The effects of atmospheres of CO₂ and N₂ on the lower critical solution temperature (LCST) of poly(N-isopropylacrylamide) (PNIPAAm) in aqueous solution have been investigated using high-pressure differential scanning calorimetry (HP-DSC). In the absence of CO₂ and N₂, the phase transition—from the hydrated to dehydrated state—of PNIPAAm in aqueous solution was characterized by an endothermic peak near 30.5 °C, namely the LCST. This endothermic peak shifted to relatively lower temperature upon increasing the pressure of CO₂, but shifted slightly to higher temperature under a higher pressure of N₂. This behavior appears to be associated with the CO₂ molecules displacing the H₂O molecules from around the amide groups of PNIPAAm upon increasing the pressure, thereby enhancing the formation of intramolecular hydrogen bonds between the amide groups; in contrast, increasing the N₂ pressure strengthened the interactions of H₂O with the apolar isopropyl and amide groups. Despite the difference in the effects of CO₂ and N₂ on the LCST, higher pressures of CO₂ and N₂ both led to more positive changes in enthalpy (ΔH) for the phase transition per mole of NIPAAm units. The higher values of ΔH at higher CO₂ pressures presumably resulted from the formation of strong intramolecular hydrogen bonds. For a given CO₂ pressure, the value of ΔH was less positive at higher concentrations of PNIPAAm, suggesting a lesser degree of disruption of hydrogen bonds during its HP-DSC heating scans. Under a given pressure of CO₂, the addition of a salt (NaCl, KCl, KBr) led to a further decrease in the LCST and the value of ΔH of the aqueous PNIPAAm solution, due to the salt ions coordinating H₂O molecules.